Miniature laser projectors, which have been referred to as pico projectors, nano projectors, and micro projectors, are generally battery operated portable projectors that have been gaining popularity because their small size makes them convenient for certain applications. The devices can be connected to handheld devices or laptops and can display images on screens or walls.
Unfortunately, miniature laser projectors generally exhibit low light output, thereby limiting their utility to environments with low ambient background light. These laser-based units typical display power in the order of only 1 mW.
Maximum brightness for these devices can be increased by increasing the power of the output; however, such increases in power can be detrimental. From a health perspective, increases in laser power can augment the possibly of damage to eyes. From an operational perspective, increases in laser power can drain battery power rapidly and create excessive heat that must be dissipated to avoid damaging the laser diode or changing the operational performance.
Modulation of the horizontal scan velocity in the fast scan axis has been considered to improved peak bright in miniature displays, which had been incorporated in CRT displays to improve the apparent sharpness of edges; however, this was not used to increase brightness.
In conventional laser projectors, a moving micromirror is used to raster scan the laser beam in a manner that is analogous to the steering of electron beams in cathode ray tubes. The horizontal scan motion is created by running the horizontal axis at its resonant frequency, which is typically about 18 KHz. The horizontal scan velocity varies sinusoidally with position. A scan controller uses feedback from sensors on the scanner to keep the system on resonance and at a fixed scan amplitude. The image is drawn in both directions as the scanner sweeps the beam back and forth. This helps the system efficiency in two ways. First, by running on resonance, the power required to drive the scan mirror is minimized. Second, bi-directional horizontal scanning of video maximizes the laser use efficiency by minimizing the video blanking interval. Bi-directional implies that the laser is emitting light during both left and right sweeps of the laser beam (or up and down sweeps if the raster scan pattern is rotated). This results in a brighter projector for any given laser output power.
The vertical scan direction is traditionally driven with a standard sawtooth waveform to provide constant velocity from the top to the bottom of the image and a rapid retrace back to the top to begin a new frame as shown in FIG. 1. The frame rate is typically 60 Hz for an 848×480 WVGA resolution; it can be increased under some conditions or some particular application requirements.
In light of the limited utility of miniature laser displays and possible safety and performance concerns associated increasing laser power, a need exists to more optimally operate miniature laser displays.